In some aspects of the invention, a zero current detecting circuit of a switching power supply device detects a gradient of the current flowing in an inductor in the OFF state of the switching element and detects the timing at which the current through the inductor becomes zero corresponding to the detected gradient of the inductor current. Specifically, the zero current detecting circuit receives a signal for controlling ON/OFF driving of the switching element and a voltage signal proportional to the current flowing through the inductor in an OFF state of the switching element. The voltage signal can be compared sequentially with first and second comparison reference voltages to control charging and discharging of a capacitor. Further, the zero current detecting circuit can detect a timing at which the charging and discharging voltage of the capacitor as the timing of zero current flowing through the inductor.